Liquid crystal display units have hitherto found preferential application in quartz timepieces and pocket calculators. Since liquid crystal display units consume hardly any power in operation and therefore require a minimal power supply, they are finding increasing application in communications equipment as well.
In liquid crystal display units so-called "twisted nematic liquid crystals" are contained between a front and rear polarization layer. The molecules of the mesomorphous bonds are elongated, and in the nematic phase, disregarding heat fluctuations, the longitudinal axes of the molecules are in parallel alignment. When an electric field is applied to the liquid crystal, so-called Freedericksz transitions take place, as a result of which, given a suitable arrangement of the polarizers, the initially opaque liquid becomes transparent.
In the absence of an external electric field, the direction of the axes of each molecular layer is twisted about a small angle relative to the neighbouring molecular layer. When the twisted planar liquid crystal layer is exposed to light which, at the input side, is linearly polarized along the mean direction of the longitudinal axes of the molecules, the plane of polarization in the layer rotates in accordance with the twisted structure of the molecular layers. If the direction of polarization of the rear polarizer is the same as the mean direction of the longitudinal axes of the molecules, correspondingly polarized light is able to enter the liquid crystal display unit and is rotated in correspondence with the twist of the planar layers. If for example the light (daylight or light from a lamp) enters from the rear with horizontal polarization, it is vertically polarized when it emerges from the liquid crystal layer. If the front polarizer is likewise vertically oriented, the light will pass through it unhindered and the image surface appears bright to the observer.
In liquid crystal display units the liquid-crystalline layer is sandwiched between rear and front glass plates (and between a seal encapsulating the glass plates), which are provided with transparent electrodes. When a voltage is applied to the two electrodes the liquid-crystalline layer is exposed to an electric field which orients all molecular layers in the direction of the field, thereby removing the twist in the liquid crystalline layer and hence the rotation of the direction of polarization of the light passing through. When horizontally polarized light enters from the rear, it encounters the vertically oriented front polarizer, which does not allow it to pass. The observer thus sees the image surface as dark.
For displaying symbols, graphs and pictures it is common practice to use a matrix display, i.e. a display with image elements arranged horizontally and vertically parallel to one another. To display a set of particular characters or symbols, e.g. numbers, it is also common practice, however, to use a segment display, e.g. 7-segment display. For addressing or selecting the image elements and controlling their brightness, a control circuit is connected to the liquid crystal display unit. The brightness control is frequently performed by means of pulse-amplitude modulation, that is to say the amplitude of the control pulses determines the light transmission and hence the brightness of the image element. In multiplex control with a high multiplex ratio (e.g. 1:8) the line and column electrodes are driven with short pulses, whereby the difference in voltage amplitude between the segments (or image points) switched on and those switched off decreases with increasing multiplex ratio, resulting in a low contrast ratio in the liquid crystal display unit.
As can be seen from the isocontrast characteristics of liquid crystal display units, the recognizability of the information depends on the range of angles from which the display is observed. An isocontrast characteristic is a line in a polar diagram which indicates the direction in space and in which the contrast of the liquid crystal display unit has a constant value. The contrast ratio changes depending on the orientation of the liquid crystal unit (expressed by the angles alpha and phi).
A liquid crystal display unit for a motorized vehicle is known from German Offenlegungsschrift No. DE-PS 30 29 122. The transmissive display unit is fitted at the rear with a light-conducting body, which is movable to allow optionally for exposing the liquid crystal layer to daylight or to light from an artificial light source. In the daytime setting the light source is switched off and the daylight is beamed through a lens to illuminate the light-conducting body. This light-conducting body is coated at the rear with a reflecting layer, which causes the incident daylight to illuminate the liquid crystalline layer from the rear. In the nightime setting the light-conducting body lies against the rear of the liquid crystal display unit and the light source is switched on. The light reflected from the coating of the light-conducting body serves for illuminating the liquid crystal display unit. The brightness of the liquid crystal display unit known from the above-mentioned German Offenlegungsschrift No. 30 29 122 is controlled by the brightness of ambient light or by the luminous flux of a light source. These values are chosen in such a way that the information is readily recognizable within a prescribed range of observation angles.